1. Field of the Invention
This invention relates to an information processing apparatus incorporating a file memory device, and particularly to a file memory device suitable for the speed-up of file access and to an information processing apparatus using it.
2. Description of the Related Art
The file memory is an almost indispensable peripheral device for general-purpose information processing systems such as personal computers. Generally, file memories are built into the main unit of the information processing system, thereby allowing the user to deal with capacious files.
Recently, notebook and palm-type personal computers have gained popularity, their appeal lying principally on their usefulness in terms of portability. Accordingly, file-memory-based semiconductor memory chips have been used in place of magnetic-type disk memory because the latter is not ideally suited to a notebook computer environment, i.e. They are not reliable against vibrations and consume too much power. An example of a system which employs a semiconductor file memory based on a flash memory is disclosed in Japanese patent publication JP-A-2-292798.
A flash memory is an electrically erasable and programmable non-volatile memory. Because they can be manufactured at comparatively low cost in large-scale production, flash memories have proven to be one of the most effective storage mediums for use as a semiconductor file memory. The technique of the above-mentioned patent publication is intended to solve many of the problems that are encountered in designing a file memory based on the flash memory. Specifically, the cited patent publication recognizes that frequent erasing operations in a file memory causes damage to many of its file memory elements. The cited patent proposes a method of alleviating this drawback of the file memory, and in addition proposes a method of speeding up erasing operations required for rewriting data therein. The semiconductor file memory further achieves compatibility with the magnetic disk memory with respect to the way in which it interfaces with main information processing apparatus; with the principal intention of reorganizing information processing systems by replacing their magnetic disk memories with semiconductor memories.
The above-mentioned prior art semiconductor file memory achieves compatibility with magnetic disk memories by using the existing interface bus of the main information processing apparatus. Although this design principle allows the user to easily accept the semiconductor memory, because of its compatibility with the magnetic disk memory, it does not take advantage of the superiority which the semiconductor memory has over magnetic disk memories. For example, a semiconductor memory in the form of a static storage medium enables very fast data access. The magnetic disk memory, in contrast, reads or writes data at certain positions on a turning disk. This fast access property of the semiconductor memory, however, cannot be utilized with the interface that is designed for the magnetic disk memory.
Magnetic disk memories used in presently existing information processing apparatuses such as personal computers are slow in data access relative to the main memory access. Therefore, the magnetic disk memory does not need to operate in synchronism with the CPU of processing apparatus, and it transacts data over an asynchronous data bus. In contrast, semiconductor memories are fast enough to operate in synchronism the CPU. The ability of a file memory to operate synchronously with a CPU becomes significant.
However, if it is intended to overcome the difference of the data bus width of a flash memory chip from the CPU data bus width by using memory chips in parallel, a new problem arises. Namely, a flash memory has a fixed size of unit erasure block area, which is typically 512 bytes. Accordingly, when multiple memory chips are used in parallel, an area equal to the unit block area (e.g., 512 bytes) multiplied by the number of chips in parallel in erased at once.
Many personal computers have a unit storage data block for file management (i.e., a sector having a size) of 512 bytes. Thus, for example, if it is intended to use four flash memory chips in parallel, a rewrite access to one file sector will result in the erasure of an area that is four times the sector. This unit erasure block size is too large, and unintentional erasure of other data can occur.